This invention relates generally to neutronic reactors, and more specifically to electric control circuits controlling the operation of safety devices for preventing neutronic chain reactors from exceeding safe operating limits.
All neutronic reactors are constructed with as much excess reactivity as is possible considering the available reactor control system, the excess reactivity of a neutronic reactor being defined as the amount that the reproduction ratio of a neutronic reactor under most favorable conditions exceeds unity. The reproduction ratio of a neutronic reactor is the ratio of the number of neutrons in any given generation to the number of neutrons in the preceding generation within the actual pile structure. The excess reactivity of a neutronic reactor determines the magnitude of isotope production and other neutron-absorbing activities which the reactor may accomplish, and also determines the flexibility which is possible in operating the reactor.
The existence of excess reactivity in a reactor makes desirable both regulating control means and safety control means for the reactor. Regulating control means are necessary in order to maintain the reproduction ratio of the reactor at unity during constant power operation, and to make adjustments in the power level of the reactor. Safety control means are desirable in order to shut down the reactor more rapidly when unsafe operating conditions develop than is possible with the regulating control means.
There are many causes of unsafe operating conditions which make it desirable to shut down a neutronic reactor. If the reactor xe2x80x9cperiodxe2x80x9d becomes too short for any reason, it is desirable to shut down the reactor, the reactor period being the time required for the neutron flux density within the reactor to increase by a factor of e, or 2.718. In those reactors which employ cooling means and operate at substantial power levels, it is also desirable to shut down the reaction if there is a decrease in the flow of the coolant. There are also many other reasons for providing a safety control system to shut down a neutronic reactor, and the safety control system may be coupled to any of these dangerous conditions.
A number of safety control systems have been developed in the neutronic reactor art. In one of these systems, neutron-absorbing rods are disposed within channels which extend into the active portion of the reactor, the active portion being the region of the reactor in which the fissionable material is disposed. The rods of neutron-absorbing materials are mechanically biased to enter the active portion of the reactor when released, either by the attraction of gravity or some impelling force. This invention pertains in particular to the safety system disclosed in a copending U.S. patent application Ser. No. 413,735 of common assignee filed Mar. 2, 1954 in the name of Albert T. Strand.
An object of the present invention is to provide a safety system which provides two separate safety operations, the failure of one operation actuating the second operation, thereby greatly reducing the probability of a failure of the safety system.
Another object of the present invention is to provide a simple but accurately responsive control circuit for controlling the safety system.
A further object of the present invention is to provide a control circuit which will function reliably without being dependent on physical contact with the rest of the safety system.